Dwarf Planet
A dwarf planet is a round body orbiting the Sun that has not cleared its orbital neighborhood; Pluto, Ceres, and Eris are examples.
Definition
A dwarf planet is a round world that orbits the Sun but is too small to sweep other objects out of its path; Pluto is the best known, and dwarf planets are real worlds with mountains, ice, and sometimes moons. Formally, a dwarf planet orbits the Sun, is massive enough for gravity to make it round, has not cleared its orbital neighborhood, and is not a moon. The IAU recognizes five: Ceres in the asteroid belt, and Pluto, Eris, Haumea, and Makemake in the Kuiper belt region, with many more expected. Ceres is a differentiated body with a water-rich mantle, the trans-Neptunian dwarf planets are ice-rock bodies with surfaces of nitrogen, methane, and water ice, and Pluto sits in a $2{:}3$ resonance with Neptune, completing two orbits for every three of Neptune's, which prevents close encounters even though the orbits cross.
Example
Pluto has five moons and mountains of water ice, and Ceres, the largest object in the asteroid belt, is also a dwarf planet. Pluto's diameter is about $2{,}380\ \text{km}$, smaller than Earth's Moon, and it orbits once every $248$ years at an average of about $39$ AU on a tilted, stretched path that sometimes brings it inside Neptune's orbit. New Horizons flew past Pluto in July 2015 and found Sputnik Planitia, a nitrogen-ice plain with no craters, indicating a surface renewed within the last few million years by convection in the ice.
Key Insight
Pluto did not shrink or change when it was reclassified in 2006; scientists learned it had many neighbors in the Kuiper belt and updated the definition of planet. Eris, discovered in 2005, is slightly more massive than Pluto and forced the question of why Pluto should count and Eris should not. The category reflects a real physical difference: Pluto's share of the mass in its orbital zone is a tiny fraction, while each of the eight planets dominates its zone by a factor of thousands or more.